7000-series aluminum alloy extruded product and method of producing the same

ABSTRACT

A method of producing an aluminum alloy extruded product comprising: casting a billet using a 7000-series aluminum alloy having Mg of 0.95 to 1.95 mass %, Zn of 5.10 to 7.90 mass %, an excess mg or Zn content relative to a stoichiometric composition shown by MgZn2 of less than 0.5 mass %; an A=Zn−5.36×Mg (mass %) of −2.64 to 0.50; and at least one of Mn, Cr and Zr with a total content thereof being less than 0.25 mass %; homogenizing the billet at 450 to 550° C.; preheating the homogenized product at 480 to 540° C.; extruding the preheated product with a die heated at 440 to 500° C.; and subjecting the extruded product to press quenching at an air cooling rate of 29° C./min or more, wherein a proof stress increase due to natural aging is 15 Mpa or less and a hardness HV increase due to natural aging is seven or less.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 12/254,348 filed Oct. 20, 2008, which is a continuation of International Patent Application No. PCT/JP2008/055408, having an international filing date of Mar. 24, 2008, which designated the United States, the entirety of which is incorporated herein by reference. Japanese Patent Application No. 2007-080296 filed on Mar. 26, 2007 is also incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to a 7000-series aluminum alloy extruded product that advantageously suppresses a positive effect due to natural aging even when the aluminum alloy is allowed to stand at room temperature after extrusion and then subjected to artificial aging, as compared with the case of subjecting the aluminum alloy to artificial aging immediately after extrusion, and a method of producing the same.

An extruded product produced using a 7000-series aluminum alloy is generally subjected to artificial aging after extrusion to obtain desired mechanical properties.

When producing an automotive structural member using such an extruded product, the extruded product is generally subjected to secondary processing (e.g., bending) in a state in which the proof stress is low (i.e., before artificial aging), and then subjected to artificial aging.

However, an automotive structural member may be required to have an impact energy absorption within a given range.

For example, when an automotive bumper reinforcement member has high strength, but exhibits a low energy absorption during side impact, the automobile is deformed to a large extent. As a result, the repair cost may increase, or the safety may be impaired.

A related-art 7000-series aluminum alloy extruded product shows an increase in proof stress after artificial aging when the extruded product is allowed to stand at room temperature after extrusion. Therefore, cracks tend to occur during side impact even if the proof stress is high so that the impact resistance (toughness) decreases.

In this case, secondary processing (e.g., bending) must be completed immediately after extrusion. This makes process management difficult.

Japanese Patent No. 3772962 discloses an automotive bumper reinforcement member made of a 7000-series aluminum alloy. When using the 7000-series aluminum alloy disclosed in Japanese Patent No. 3772962, transition elements such as Mn, Cr, and Zr must be added to obtain a fiber internal structure. Moreover, since overaging is required, the hardenability (quench sensitivity) must be taken into consideration. Therefore, the proof stress may not increase depending on the cross section of the extruded product. This complicates the production process so that the production cost increases.

SUMMARY

According to one aspect of the invention, there is provided an aluminum alloy extruded product comprising a 7000-series aluminum alloy, the 7000-series aluminum alloy having an excess Mg content or an excess Zn content with respect to a stoichiometric composition shown by MgZn₂ of less than 0.5 mass %.

According to another aspect of the invention, there is provided a method of producing an aluminum alloy extruded product, the method comprising homogenizing a billet that is cast using the 7000-series aluminum alloy as defined in claim 1 at 450 to 550° C., preheating the homogenized product at 480 to 540° C., extruding the preheated product, and subjecting the extruded product to press quenching at a cooling rate of 29° C./min or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an aluminum alloy composition.

FIG. 2 shows evaluation results for an aluminum alloy extruded product.

FIG. 3 shows an example of a double hollow cross section of an aluminum alloy extruded product according to one aspect of the invention.

FIGS. 4A and 4B show an example of a triple hollow cross section of an aluminum alloy extruded product according to one aspect of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention may provide a 7000-series aluminum alloy extruded product of which an increase in proof stress or the like due to natural aging is suppressed by suppressing the positive effect due to storage at room temperature after extrusion, and a method of producing the same.

According to one embodiment of the invention, there is provided an aluminum alloy extruded product comprising a 7000-series aluminum alloy according to the Japanese Industrial Standards (JIS), the 7000-series aluminum alloy having an excess Mg content or an excess Zn content with respect to a stoichiometric composition shown by MgZn₂ of less than 0.5 mass %.

Note that the stoichiometric composition shown by MgZn₂ means that the ratio of components added is MgZn₂, and does not necessarily mean that the precipitate is MgZn₂.

In the aluminum alloy extruded product, the aluminum alloy extruded product may have an Mg content of 0.95 to 1.95 mass % and a Zn content of 5.10 to 7.90 mass %.

The alloy may be designed so that the Mg content and the Zn content are within the above ranges, and the value A=Zn−5.36×Mg (mass %) is −2.64 to 0.50.

In one embodiment of the invention, other components may optionally be added to the aluminum alloy insofar as the aluminum alloy is an Al—Zn—Mg alloy containing aluminum as a base metal.

In the aluminum alloy extruded product, an increase in proof stress due to natural aging may be 15 MPa or less when comparing the proof stress of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 50° C. or less for one week after extrusion and then subjecting the resulting product to artificial aging with the proof stress of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging immediately after extrusion.

In the aluminum alloy extruded product, an increase in hardness HV due to natural aging may be seven or less when comparing the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 50° C. or less for one week after extrusion and then subjecting the resulting product to artificial aging with the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging immediately after extrusion.

Note that HV indicates Vickers hardness.

According to another embodiment of the invention, there is provided a method of producing an aluminum alloy extruded product, the method comprising homogenizing a billet that is cast using the 7000-series aluminum alloy as defined in claim 1 at 450 to 550° C., preheating the homogenized product at 480 to 540° C., extruding the preheated product, and subjecting the extruded product to press quenching at a cooling rate of 29° C./min or more.

When heating a cylindrical billet to a given temperature and directly or indirectly extruding the billet using an extrusion press, a high-temperature extruded product is extruded from an extrusion die. The term “press quenching” used herein refers to cooling the extruded product using a fan or the like to achieve effects of quench (quenching effects).

In order to achieve sufficient effects of quench, it is preferable that the preheating temperature of the billet be set at 480° C. or more and the cooling rate after extrusion be set at 29° C./min or more.

Each component of the aluminum alloy is described below.

Zn and Mg

Zn and Mg are bonded to improve the proof stress due to precipitation hardening.

Therefore, the Zn content and the Mg content are designed corresponding to the required proof stress. One aspect of the invention is characterized in that the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ is less than 0.5 mass %.

When designing the Mg content and the Zn content as described above, it is particularly effective to set the Mg content at 0.95 to 1.95 mass % and set the Zn content at 5.10 to 7.90 mass %.

In this case, it is preferable that the value A=Zn−5.36×Mg be −2.64 to 0.50, taking the atomic weights of Mg and Zn into consideration.

Cu

Cu reduces the potential difference between the grain boundary and the inside of the grain with a small amount of addition to improve the stress corrosion cracking resistance. Cu also improves the proof stress.

If the Cu content exceeds 0.4 mass %, the extrudability and the corrosion resistance deteriorate.

The Cu content is preferably 0.3 mass % or less from the viewpoint of corrosion resistance.

Mn, Cr, and Zr

Mn, Cr, and Zr are bonded to Al to form minute compounds to suppress recrystallization so that a fiber structure can be obtained.

Although each of Mn, Cr, and Zr serves as a fiber structure-forming element, it is effective to add these elements in combination. In particular, it is preferable to add Zr in an amount greater than those of Mn and Cr from the viewpoint of suppressing recrystallization. It is necessary to control the content of each of these elements to less than 0.25 mass %. If the total content of these elements exceeds 0.25 mass %, the hardenability increases so that a sufficient strength cannot be obtained by air cooling. Moreover, the size of compounds increases so that the toughness deteriorates.

Fe

Fe is an unavoidable impurity. Fe is bonded to Al and Si to form an Al—Fe—Si compound, or is bonded to Al to form an Al—Fe compound.

Such a compound tends to serve as a breakage starting point to decrease the toughness. Therefore, the Fe content is 0.35 mass % or less, and preferably 0.20 mass % or less.

Si

Si is an unavoidable impurity. Si is bonded to Al and Fe to form an Al—Fe—Si compound.

Such a compound tends to serve as a breakage starting point to decrease the toughness. Therefore, the Si content is 0.1 mass % or less, and preferably 0.05 mass % or less.

Homogenization of Billet

A billet is homogenized to eliminate segregation of the main components (e.g., Mg, Zn, and Cu) in the billet and to divide and reduce the size of coarse Mn, Cr, Zr, Fe, and Si compounds that are crystallized during casting to decrease the toughness.

The homogenization temperature differs depending on the aluminum alloy components (alloy series). The solution treatment temperature suitable for a 7000-series Al—Zn—Mg alloy is 450 to 550° C.

It is preferable that the homogenization temperature of the billet be high, preferably 480° C. or more, and ideally 520° C. or more, while controlling the total content of elements (e.g., Mn, Cr, and Zr) that tend to undergo segregation at 0.25 mass % or less.

The upper limit of the homogenization temperature is set at 550° C. because local melting may occur if the billet is held at a temperature of more than 550° C. for a specific period of time.

If the homogenization temperature is less than 450° C., crystallized products produced when casting the billet are not sufficiently divided and reduced in size. As a result, the toughness decreases.

Extrusion Conditions

An Al—Zn—Mg high-strength aluminum alloy exhibits poor extrudability as compared with a 6000-series alloy. Therefore, the extrusion conditions are also important factors.

The heating temperature of the billet is preferably 480 to 540° C. If the heating temperature is less than 480° C., the billet may not be extruded due to high extrusion resistance. If the heating temperature exceeds 540° C., the proof stress tends to decrease.

The temperature of the extrusion die is preferably 440 to 500° C. If the temperature of the extrusion die is less than 440° C., the billet may not be extruded due to a decrease in material temperature. If the temperature of the extrusion die exceeds 500° C., the die tends to break during annealing.

The temperature of the extruded product immediately after extrusion is preferably 580° C. or less. If the temperature of the extruded product exceeds 580° C., a pickup occurs on the surface of the extruded product, whereby the appearance may deteriorate.

Hollow Cross-Sectional Shape of Extruded Product

FIGS. 3, 4A and 4B show cross section examples used for evaluation tests.

A double hollow cross section shown in FIG. 3 has a dimension a of 70 to 150 mm, a dimension b of 50 to 100 mm, and a thickness t of 1 to 6 mm.

A triple hollow cross section shown in FIG. 4A has a dimension a of 40 mm<a≦75 mm, a dimension b of b≦120 mm, and rib thicknesses of 3≦t₁≦8, 1≦t₂≦6, 1≦t₃₁≦6, and 1≦t₃₂≦6.

A cross section shown in FIG. 4B has a dimension a of a≦40 mm, a dimension b of b≦140 mm, and rib thicknesses of 3≦t₁≦8, 1≦t₂≦6, 1≦t₃₁≦6, and 1≦t₃₂≦6.

Note that FIGS. 4A and 4B show schematic cross sections. An upright rib may be provided outside the peripheral rib.

The cross sections shown in FIGS. 3, 4A and 4B are examples of the cross section of a bumper reinforcement member provided on the front side and the rear side of an automobile.

The side impact energy absorption during collision is increased by forming a bumper reinforcement member having a double hollow cross section or a triple hollow cross section.

Moreover, cracks rarely occur during side impact so that the toughness increases.

In one embodiment of the invention, the content of Mg and Zn as the main components of the 7000-series aluminum alloy are set so that the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ is less than 0.5 mass %. Therefore, a positive effect due to storage at room temperature can be suppressed so that a decrease in side impact energy absorption can be suppressed.

Moreover, the time management from extrusion to secondary processing is facilitated.

Examples 1 to 7

Molten metal having the composition shown in FIG. 1 (table) was prepared, and was cast into a cylindrical billet with a diameter of 204 mm. The billet was homogenized at 480 to 520° C. for about 12 hours or more.

The value of each component shown in FIG. 1 indicates an analytical value or a significant value calculated from the analytical value.

Extruded products having a double hollow cross section shown in FIG. 3 and extruded products having a triple hollow cross section shown in FIGS. 4A and 4B were air-cooled using a fan immediately after extrusion, subjected to press quenching, and subjected to two-stage artificial aging (90° C.×4 hours and 140° C.×14 hours), or subjected to artificial aging (90° C.×4 hours and 140° C.×14 hours) after natural aging at 40 (i.e., 50° C. or less) for one week (seven days) to obtain specimens. FIG. 2 (table) shows the 0.2% proof stress (significant value) and the Vickers hardness HV (significant value) (load: 5 kg) of each specimen.

A specimen for measuring the 0.2% proof stress was prepared based on a JIS Z 2201 metal material tensile test specimen, and the 0.2% proof stress was evaluated in accordance with JIS Z 2241 “Metal Material Tensile Test Method”.

The Vickers hardness HV was evaluated in accordance with JIS Z 2244 “Vickers Hardness Test Method”.

Examples 1 to 7 indicate aluminum alloy extruded products according to the examples of the invention. Comparative Examples 1 to 11 are provided to clarify the characteristics of the aluminum alloy extruded products according to Examples 1 to 7 of the invention.

In the table, an Mg content of 0.95 to 1.95 is indicated as “Good”, and a Zn content of 5.10 to 7.90 is indicated as “Good”.

A value A=Zn−5.36×Mg of −2.64≦A≦0.50 is indicated as “Good”, an increase in 0.2% proof stress of 15 MPa or less is indicated as “Good”, and an increase in hardness HV (load: 5 kg) of 7 or less is indicated as “Good”.

The amount of MgZn₂ added was 6.38% in Example 1, 7.95% in Example 2, and 8.90% in Example 3. The proof stress increased along with an increase in the amount of MgZn₂ added.

This tendency was also observed for the comparative examples. However, when comparing Example 1 with Comparative Examples 1, 2, 3, 4, and 8, an increase in proof stress due to natural aging was 9 MPa (i.e., 15 MPa or less) in Example 1 in which the excess Zn content (+exZn) was 0.02%. On the other hand, an increase in proof stress due to natural aging was more than 15 MPa in Comparative Examples 1, 2, 3, 4, and 8.

An increase in hardness HV due to natural aging was four (i.e., seven or less) in Example 1. On the other hand, an increase in hardness HV due to natural aging was 10 or more in Comparative Examples 1, 2, 3, 4, and 8.

Example 2 indicates a composition in which Zn and Mg were balanced. In Example 3 in which the excess Mg content (+exMg) was 0.41%, an increase in proof stress due to natural aging was 15 MPa or less, and an increase in hardness HV due to natural aging was seven or less.

In Comparative Examples 5, 6, and 7 in which the Zn content was increased to 5.40% (i.e., the Mg content was decreased), an increase in proof stress due to natural aging was more than 15 MPa.

In Examples 4 to 7, the Mg content was set at 0.95 to 1.95 and the Zn content was set at 5.10 to 7.90, and the relationship between the value A=Zn−5.36×Mg and the positive effect due to natural aging was investigated while setting the excess Mg content or the excess Zn content with respect to the stoichiometric composition shown by MgZn₂ at less than 0.5 mass %.

When the value A was −2.64 to 0.50, an increase in proof stress due to natural aging (40° C.×7 days) was 15 MPa or less, and an increase in hardness HV due to natural aging was seven or less.

In Comparative Example 8 in which the Mg content and the Zn content were within the design ranges, but the excess Mg content was 0.72 mass % (i.e., 0.5 mass % or more) and the value A was −3.86 (i.e., −2.64 or less), an increase in proof stress was 16 MPa and an increase in hardness HV was 11 (i.e., the target values of the examples of the invention were exceeded).

In Comparative Examples 9, 10, and 11, when the excess Mg content or the excess Zn content was less than 0.5 mass %, but the Mg content was 5.10% or less or the Zn content was 0.95% or less, an increase in proof stress and an increase in hardness HV exceeded the target values of the examples of the invention. Therefore, it was found that it is preferable to set the Mg content and the Zn content within the above-mentioned ranges, and set the amount of MgZn₂ at 5.4% or more, and preferably 6.0% or more.

In the examples of the invention, the difference due to natural aging at 40° C. for one week was evaluated by the proof stress value and the hardness. Since it was confirmed that the positive effect due to natural aging is suppressed, it is considered that the toughness is stabilized due to artificial aging so that the impact resistance increases.

Since the aluminum alloy extruded products according the examples of the invention can suppress the positive effect due to artificial aging after extrusion, the artificial aging effect after secondary processing is stabilized even if the extruded product is allowed to stand at room temperature for a long period of time. Therefore, the aluminum alloy extruded products can be widely used as 7000-series aluminum alloy extruded products utilized in the field in which the required quality is strictly limited to a narrow range, such as automotive bumper reinforcement members.

Although only some embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within scope of this invention. 

1. A method of producing an aluminum alloy extruded product, the method comprising: casting a billet using a 7000-series aluminum alloy having an Mg content of 0.95 to 1.95 mass %, a Zn content of 5.10 to 7.90 mass %, an excess mg content or an excess Zn content with respect to a stoichiometric composition shown by MgZn2 of less than 0.5 mass %; a value A indicated by a relational expression A=Zn−5.36×Mg (mass %) of −2.64 to 0.50; and at least one of Mn, Cr, and Zr, a total content of Mn, Cr, and Zr being less than 0.25 mass %; homogenizing the billet at 450 to 550° C.; preheating the homogenized product at 480 to 540° C.; extruding the preheated product with an extrusion die that is heated at 440 to 500° C.; and subjecting the extruded product to press quenching at an air cooling rate of 29° C./min or more, wherein an increase in proof stress due to natural aging is 15 Mpa or less and an increase in hardness HV due to natural aging is seven or less, when comparing the proof stress and the hardness of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to natural aging at 50° C. or less for one week after extrusion and then subjecting the resulting product to artificial aging with the proof stress of the aluminum alloy extruded product obtained by subjecting the aluminum alloy to artificial aging immediately after extrusion.
 2. The method as defined in claim 1, wherein the extruded product immediately after extrusion is 580° C. or less. 